usb: mtu3: rebuild role switch get/set hooks

Use common helper ssusb_set_mode() to do role switch instead
of manual switch helper;
Remove unnecessary local variable when get role status

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1623139069-8173-14-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>